Nuclear magnetic resonance (NMR) offers the opportunity of selectively examining the nature and function of nucleii of atoms attached to a variety of important biochemicals, of which phosphorus and carbon appear to be currently the most useful, and are pivotal in cell metabolism as well. Nuclear magnetic resonance can most simply be described as a pulsed nuclear clock, the rate of which is sensitively dependent upon the chemical environment of the particular nucleus. For example, the atoms of phosphorus, which are attached to a series of key energy-related compounds of the body, give an appropriate signature, where the important phosphate compounds in the brain, heart, kidney, liver, and skeletal tissues are the high energy compounds, ATP, the "energy currency" of the body, and creatine phosphate, PCr, the "short-term energy reserve" of the body, together with low energy forms of these compounds, adenosine diphosphate and inorganic phosphate. In addition, the sugar phosphate from the metabolic pathway activated by glucose metabolism can also be found (F6P, DPG).
In the past there have been numerous problems in applying NMR to tissue under the proper conditions, and in collecting and interpreting the resultant electrical data. In particular, previously proposed methods for determining the relationship between work output from an exercising body member, using the NMR technique, have been unsatisfactory because they do not employ a reliable means for accurately measuring the work output.